The traditional process for paving roadways with asphalt material is generally carried out by an asphalt paving machine and a number of supply trucks which transport the asphalt material from an asphalt production plant to the paving machine. The paving machine generally is self-propelled and driven by a wheeled or tracked drive system. A hopper is located at the front end of the machine to receive asphalt material from a truck, and a floating screed is located at the rear end of the machine to form the asphalt mat. A conveyor system typically comprised of slat conveyors and screw augers delivers the asphalt material from the hopper to the road base just ahead of the screed.
A typical asphalt paving machine has a hopper with a capacity of 5-15 tons, whereas a typical dump-type delivery truck has a capacity of about 20 tons. The front of the paving machine is usually provided with rollers which are adapted to engage the rear tires of a delivery truck. This arrangement enables asphalt material to be transferred from the truck to the asphalt paving machine by positioning the delivery truck in front of the paving machine and raising the dump bed of the truck to dump the asphalt material into the hopper as the paving machine pushes the truck along in front of it. Because the delivery truck usually carries more asphalt material than the hopper can receive at one time, the paving machine may push the delivery truck along for several minutes while its conveyor system transports the asphalt material out of the hopper to the roadway in front of the screed.
Sometimes, problems may arise when operating the paving machine and the delivery trucks in this manner. Because of traffic conditions and other unforeseen delays, it is not uncommon for the paving machine to empty its hopper of asphalt material before a loaded delivery truck is available to begin dumping its asphalt material into the hopper. When this occurs, the paving machine must stop paving and await the arrival of another delivery truck. Even if one or more loaded delivery trucks are available to dump asphalt material into the paving machine hopper, it may be necessary to stop the paving machine. Sometimes, it is simply impossible for the truck drivers to remove an empty delivery truck from engagement with the front of the paving machine and to maneuver a loaded truck into position to dump into the hopper before the hopper is emptied.
As is known to those having ordinary skill in the art to which the invention relates, when a paving machine stops, even for a short time, the screed will tend to settle into the freshly laid asphalt mat. Then, when the paving machine resumes its forward motion, the screed will tend to ride upwardly momentarily, thus depositing an excessive amount of asphalt material on the roadway. Consequently, stopping of the paving machine causes a depression and a bump in the surface of the asphalt mat, resulting in an uneven pavement surface. Therefore, in recent years, material transfer vehicles have been employed to shuttle asphalt material between the delivery trucks and the paving machine. Such a material transfer vehicle is described in various embodiments in U.S. Pat. Nos. 4,818,139, 5,015,120, 5,035,534 and U.S. Pat. No. 7,160,056, which are incorporated herein by reference. These patents describe a self-propelled material transfer vehicle which includes a large-capacity truck-receiving hopper and a large-capacity truck-unloading conveyor extending from this hopper to a surge bin that is sized to hold the entire load of a delivery truck. A conveyor in the surge bin is adapted to transfer asphalt material to a paver-loading conveyor that is pivotable about an essentially vertical axis so that the transfer vehicle can be positioned alongside an asphalt paving machine that is laying an asphalt mat and rapidly discharge a truckload of asphalt material into the paver's hopper. Because of its rapid loading and unloading capabilities, the material transfer vehicle can rapidly shuttle between delivery trucks at a pick-up point and a paving machine that is laying an asphalt mat so that there is less likelihood that the paving machine will have to stop paving because of a lack of asphalt material.
It is known to provide systems for controlling the flow of asphalt material in an asphalt paving machine or a material transfer vehicle based upon devices or sensors that detect the height of material in or adjacent to the machine. These systems typically are used to insure that sufficient asphalt material is available for a paving operation. Thus, for example, U.S. Pat. No. 3,678,817 describes a system for controlling the flow of asphalt material to the distributing auger of an asphalt paving machine by employing a sensing paddle to float or move upwardly when the asphalt material in the pre-work station (immediately in front of the distributing auger) is sufficiently high to engage the paddle. A control system is operatively connected to the paddle and to a hydraulic cylinder that is connected to a movable gate adjacent to the distributing auger. The position of the gate controls the rate of delivery of asphalt material from the hopper of the paving machine to the distributing auger. U.S. Pat. No. 4,823,366 describes an asphalt paving machine having a system similar to that of U.S. Pat. No. 3,678,817, except that the sensing paddle is replaced with a non-contact sensor that is mounted on the paving machine and adapted to detect the top surface of paving material immediately to the front of the distributing auger. U.S. Pat. No. 5,100,277 describes a material transfer vehicle that is adapted to be mounted to the front end of a paving machine. The material transfer vehicle has a truck-unloading conveyor with a discharge end that is disposed above the feed hopper of the paving machine. The truck-receiving hopper of the material transfer vehicle is attached at the lower intake end of the truck-unloading conveyor, and a gate is mounted on the hopper above an opening onto the truck-unloading conveyor. The transfer of material to the paving machine is controlled by positioning the gate based upon signals received from an ultrasonic sensor that monitors the height of material in the hopper of the paving machine. U.S. Pat. No. 5,452,966 describes an asphalt paving machine having a front hopper and a belt conveyor which extends from the hopper through a tunnel to a pair of distributing augers at the rear of the machine. Two vertical gates are mounted at the rear of the tunnel, each of which is adapted to be moved upwardly and downwardly as controlled by feed sensors that detect the level of paving material on the roadway in front of the distributing augers.
Although it is known to provide a system for using gates to control a flow of asphalt material in order to provide a sufficient supply based upon the level of material in or adjacent to an asphalt paving machine or material transfer vehicle, these known systems are not adapted to maximize the efficiency of the machine or vehicle. It would be desirable if an automatic system could be provided to control the power demand of the truck-unloading conveyor of a material transfer vehicle by controlling the rate of introduction of asphalt material from the supply truck to the surge bin of the material transfer vehicle, thereby maximizing the efficiency of the material transfer vehicle.